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The legend of the leaning tower

04 Feb 2003 Robert P Crease

Historians are not sure if Galileo ever carried out experiments at the Leaning Tower of Pisa. So why, asks Robert P Crease, has the story become part of physics folklore?

Leaning Tower of Pisa

Commander David R Scott (2 August 1971, lunar surface): “Well, in my left hand I have a feather; in my right hand, a hammer. And I guess one of the reasons we got here today was because of a gentleman named Galileo, a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought: ‘Where would be a better place to confirm his findings than on the Moon?’.”

[Camera zooms in on Scott’s hands. One is holding a feather, the other a hammer. The camera pulls back to show the Falcon ­ the Apollo 15 landing craft ­ and the lunar horizon.]
Scott: “And so we thought we’d try it here for you. The feather happens to be, appropriately, a falcon feather for our Falcon. And I’ll drop the two of them here and, hopefully, they’ll hit the ground at the same time.” [Scott releases hammer and feather. They hit the ground at about the same time.]
Scott: “How about that! Mr Galileo was correct in his findings.”

How the legend started

The finding mentioned by Commander Scott, namely that objects of different mass fall at the same rate in a vacuum, is associated with a single person (Galileo) and a single place ­ the Leaning Tower of Pisa. The culprit is Vincenzio Viviani, Galileo’s secretary in the final years of his life.

We owe many of the Galilean legends to Viviani’s warm biography of the Italian scholar. One is the story of how Galileo climbed the Leaning Tower of Pisa and ­ “in the presence of other teachers and philosophers and all the students” ­ showed through repeated experiments that “the velocity of moving bodies of the same composition, but of different weights, moving through the same medium, do not attain the proportion of their weight as Aristotle decreed, but move with the same velocity”.

In his own books, Galileo uses thought experiments to argue that objects of unequal mass fall together in a vacuum. Without mentioning the Leaning Tower, he reports having “made the test” with a cannonball and a musket ball. What is perhaps surprising, however, is that Galileo found that the two balls did not quite fall together. This finding ­ coupled with the fact that Viviani’s biography is the only source to mention that the experiments were done at the Leaning Tower ­ causes most historians of science to doubt Viviani’s version of what Galileo did. They believe that the elderly and then-blind Galileo may have misremembered when speaking to his youthful assistant.

Dropping the ball

Science historians find Galileo’s early experiments with falling bodies fascinating, for several reasons. One is that Galileo was not the first. As far back as the sixth century, other scholars who doubted Aristotle’s account of motion had also experimented with falling bodies and concluded that Aristotle was wrong. They included several 16th-century Italians and one of Galileo’s predecessors as professor at Pisa.

Also intriguing is Galileo’s report, based on experiment, that balls of unequal weight do not only fall at different rates, but that the lighter one initially pulls ahead of the heavier one until the heavier catches up. In the early 1980s the science historian Thomas Settle tried to repeat Galileo’s falling-body experiments and, astonishingly, noted the same thing. He suggested that fatigue induced in the hand holding the heavier object tends to cause this hand to let go more slowly, even when the dropper believes the objects are released simultaneously.

Yet another fascinating side to Galileo’s experiments is the way that they slowly transformed from genuine scientific inquiries into public displays. After Galileo’s death, scientists including Robert Boyle and Willem ‘sGravesande built air pumps and special chambers to explore vertical fall in evacuated environments. King George III, for instance, once witnessed a demonstration involving a feather and a one-guinea coin falling together inside an evacuated tube. The popularity of such demonstrations continues to this day, featuring in many hands-on science exhibits. Indeed, the “drop stop” at the Boston Museum of Science is currently broken from overuse.

Teachers, no doubt, would call the Apollo “feather-drop” a sloppy experiment. Nobody bothered to measure the height from which the objects were released (probably 110-160 cm). Nobody cared that Scott was leaning over with his arms not parallel to the ground. Nobody measured the time of the fall (on the video it is just above 1 s). But as a demonstration it is unforgettable. The TV coverage ­ plus the fact that it has a webpage with video clip (see related links) ­ makes it possibly the most watched science demonstration ever.

The critical point

So why do falling-body experiments continue to be so popular? They were, for example, voted into the top 10 “most beautiful experiments” of all time in my recent poll of Physics World readers (September 2002 pp19­20). I think the answer is related to the fact that, as everyday experience suggests, heavier bodies do fall faster than light ones. Hammers and golf balls, for example, fall faster than feathers and ping-pong balls. Aristotle had codified this observation into an entire framework that was oriented by the everyday observations he was seeking to explain, involving an agent that exerted a force against resistance. Although this framework fails to incorporate acceleration, it is still the one that we mainly live in and that mainly works for us.

Thus we can still find it enlightening, or even surprising, to see with our very own eyes the expectations of that framework being violated. Galileo played a seminal role in transforming that framework, in developing the abstract thinking involved in the new one, and in illustrating its importance. So what if there was no original experiment? Galileo inspired an entire genre of experiments and demonstrations that allow us to change how we think and see. We might as well refer to these as the offspring of Galileo’s experiment at the Leaning Tower of Pisa.

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